1. Field of the Invention
The present invention relates generally to tanks in a fluid, and in particular to an improved cargo handling system for a tanker vessel for transporting liquid cargoes.
2. Description of the Prior Art
Tanker vessels for the transportation in bulk of liquid cargo are known in the art. See, for example, U.S. Pat. No. 2,918,032. Such tanker vessels generally comprise a plurality of liquid-tight transverse bulkheads and one or more liquid-tight longitudinal bulkheads which subdivide the tanker vessel into a plurality of liquid-tight storage compartments. If the bottom or a side of the hull of such a tanker vessel is ruptured by grounding or some other accident, the affected cargo tanks will leak until the "pressure head" of the liquid cargo in each tank, i.e., approximately the portion of the liquid cargo disposed above the waterline of the vessel, flows out of the tanker vessel. Following such leakage, it is often a significant problem and also an additional pollution potential, not to be able to remove or transfer cargo.
In recent years, pollution by oil tankers as a result of a hull rupture caused by grounding or other major catastrophe has become of increasing concern. As a result, various anti-pollution tanker constructions have been proposed. For example, it has been proposed to construct a tanker vessel with double sides defining side tanks extending from the top deck of the vessel to the hull bottom which are disposed adjacent to and associated with cargo tanks in the vessel. See U.S. Pat. No. 3,832,966. These side tanks have a volume from the hull bottom to the waterline of the tanker vessel which is equal to the respective volumes of the cargo tanks above the waterline. Valves coupling the side tanks to the cargo tanks are opened if the tanker hull is ruptured to permit oil in the cargo tanks to drain off into the side tanks so that the oil above the waterline in the cargo tanks does not escape from the ruptured hull bottom. Larger vents are used in the side tanks than in the cargo tanks to achieve this drainage from the cargo tanks to the side tanks instead of into the sea. The disadvantage of this design, however, is that the side tanks provided for receiving the "pressure head" of the liquid cargo carried in the cargo tanks is, similar to the space between the inner and outer hull bottoms in a double-bottom tanker, unavailable for the storage and transportation of cargo and, accordingly, increases the fabrication, maintenance and operating costs of the tanker vessel. Moreover, such a design is theoretical only and in practice would save not more than 2 to 3% of the liquid cargo carried by such a tanker vessel.
When emergency situations such as a hull rupture occur during the operation of a tanker vessel, very often there is little time, a minimum of available manpower, and quite often very little or no means in the vessel to aid in eliminating the problem. A tanker vessel's cargo handling system is a prime example of this. While the typical cargo handling system is expedient for normal cargo loading and discharging as well as ballast handling, it becomes virtually useless for handling cargo from those tanks whose bottoms have been ruptured and opened to the sea, since following such an event, there is a rapid loss of the cargo's pressure head and then after several minutes of pumping with the vessel's main cargo handling system, the bottom three to four inches of cargo will normally be displaced with sea water and thereafter only sea water will be pumped by the system.